1. Field of the Invention
The present invention relates to a recording apparatus and a recording position correcting method, and more specifically, to recording position corrections among a plurality of recording heads that record inks of different tones such as different colors or densities.
2. Description of the Related Art
To meet the recent need to record more colored images, some recording apparatuses comprise a plurality of recording heads on the same carriage or different carriages so that the respective recording heads can record the inks of the different tones. With such a recording apparatus, the recording heads are likely to be misaligned with respect to their ideal mounting positions owing to the insufficient accuracy of their dimensions or the insufficient accuracy with which they are mounted on the carriages. This misalignment may cause a recording position on a recording medium to deviate from the ideal one, resulting in the unwanted overlapping of the colors or a failure to allow them to overlap one another. As a result, the hue (color reproducibility) of a recorded image may change. Accordingly, corrections are desirably provided so that even if the mounting positions of the recording heads deviate from their ideal ones, each color can be recorded at the correct position during actual recording.
A correction method will be described below. That is, each recording head records a plurality of test patterns in such a manner that the recording heads use different recording timings. A user visually checks images of the recorded plurality of test patterns and determines and inputs the appropriate amount of correction. However, such a correction method imposes burdens on the user and is not sufficiently reliable.
Thus, the method described below has already been proposed as another solution. That is, a CCD sensor or the like is used to measure recorded test patterns. Then, on the basis of the results of the measurements, the amount of correction is calculated and thus automatically set. Further, Japanese Patent Application Laid-open No. 2000-238339 discloses an example that uses a reflection-type optical light-quantity sensor. The reflection-type optical light-quantity sensor enables the test patterns to be more accurately read and also enables corrections to be more accurately accomplished in a main scanning direction and a sub-scanning direction.
Here, brief description will be given of a method for using the reflection-type optical light-quantity to read test patterns to calculate the amount of correction. The reflection-type optical light-quantity sensor has a light emitting portion that emits light and a light receiving portion that receives reflected light. Then, measurement is made of the quantity of light in that part of the light emitted by the light emitting portion which is reflected and received by the light receiving portion. To accurately determine whether or not a recorded image is present, the emitted light is preferably focused on an area corresponding to the diameter of an ink dot on a recording medium. However, in this case, a small error in the mounting of the sensor results in a variation in the quantity of light reflected. Thus, in the prior art, light is emitted to a relatively large area so that the edge of a test pattern image can be estimated from the [averaged] output change of the received reflected light. The amount of correction is thus calculated.
FIG. 6 is a diagram showing test patterns on a sheet and output waveforms obtained when the test patterns are read by the reflection-type optical light-quantity sensor. In the described example, test patterns 601 and 602 are read to obtain the amount of correction. The sensor outputs a signal corresponding to the quantity of light received while moving in the direction shown by the arrows in the figure. Reference numeral 604 denotes the quantity of light received when the test patterns 601 and 602 are read. This figure indicates that a portion of the sheet in which the pattern is present reflects a smaller amount of light than a blank portion and has a smaller output value. Thus, an appropriate threshold 606 is set and a counting operation is performed during a section T1 starting with the first falling position and ending with the second falling position. Then, a count C1 obtained is used to determine the distance between the two patterns. With this measuring method, the offset of the recording position can be determined for each recording head. When an actual image is recorded, this offset is corrected to control the recording position. Thus, each color is recorded at the correct position. The hue (color reproducibility) of the recorded image is prevented from changing.
Such a reflection-type optical light-quantity sensor is also called a “reflection-type photo interrupter”. It is more inexpensive and has more applications than optical sensors such as CCDs. Accordingly, in recent years, it has been often applied to ink jet printers for general users and the like. The offset of the recording positions among recording heads for different colors has been automatically and accurately corrected using an inexpensive refection-type photo interrupter.
However, light emitted by the reflection-type photo interrupter has an intensity distribution around a specific wavelength. This prevents a variation in light reflected from an edge of an image recorded with a color from being distinguished from a variation in light reflected from an edge of an image recorded with another color. This may result in an error in measurement carried out by a recording apparatus that corrects a plurality of ink colors.
Referring back to FIG. 6, the above problem will be described in detail. In this figure, reference numerals 601 and 602 denote test patterns recorded using the same color ink. Reference numeral 603 denotes a test pattern recorded using a different color ink. Reference numeral 604 denotes an output waveform obtained by reading the test patterns 601 and 602. Reference numeral 605 denotes an output waveform obtained by reading the test patterns 601 and 603. The test patterns 601 and 602 are recorded using the same color ink. Those portions of the output waveforms which correspond to the presence of these test patterns have similar curves. Accordingly, the distance between both patterns can be substantially accurately measured. However, the output waveform 605 obtained by executing measurements for the different color inks has a different curve shape because of the different colors in spite of the same width possessed by both test patterns 601 and 603. Specifically, there is a difference between the level L1 of quantity of light reflected when the test pattern 601 is read (a difference in the quantity of light reflected between a blank portion and the center of the pattern) and the level L2 of quantity of light reflected when the test pattern 603 is read. A small misalignment ΔT caused by the difference between L1 and L2 occurs where the falling positions concerning different colors are determined by using the same threshold 606. This is recognized as the offset of recording position of the pattern.
With a color printer using a plurality of ink colors, the magnitude of an error such as ΔT varies with the color. Colors absorbing more emitted light allow the test pattern to be detected as a larger image.
Then, this is determined to be the offset of recording position of each recording head, leading to a measurement error. Further, a similar problem occurs if two types of inks are used which has the same color but different densities.